Signaling by the B cell antigen receptor (BCR) is induced by antigen engagement and is regulated in both quantitative and qualitative ways by dynamic alterations in the levels or activities of key negative regulatory molecules. We hypothesize that such regulation is critical for providing optimal control of antibody responses to limit autoreactivity and to promote production of high quality antibodies. Among the prominent signaling pathways activated by the BCR is activation of phospholipase Cg2, which generates the second messengers diacylglycerol (DG) and inositol trisphosphate. The proposed studies focus on regulation of DG levels in the membrane by the diacylglycerol kinases (DGKs). We have found that DGKa and DGKz, which appear to be the two principle forms of this negative regulator in lymphocytes, are upregulated as B cells mature in the spleen and are downregulated soon after antigen-induced activation of mature B cells. These and other observations suggest that dynamic regulation of DGKs plays an important role in regulation of B cell activation thresholds. We have characterized the effect of ablation of DGKa or DGKz on DG signaling in mature B cells and find that DGKz is the key regulator, while DGKa plays a secondary role. In response to a T cell-independent type 2 antigen, DGKz-/- mice make greater and faster antibody responses. Moreover, the responses of DGKz-/- B cells to a T cell-dependent antigen are also increased, especially with regard to the generation of early extrafollicular plasma cells. These studies suggest that the amount of BCR-induced DG signaling provides affinity discrimination for the early plasma cell response. The proposed studies will determine the biological significance of attenuation of DG signaling by DGKz in B cells for host defense to influenza virus infection (Specific Aim 1); will test the hypothesis that decreases in Erk signaling in B cells resulting from deletion of some but not all alleles of Erk1 and Erk2 in an allelic series will have the opposite effect on various elements of the antibody response to the effect of deletion of DGKz (Specific Aim 2); and will use deletion of some but not all alleles of Erk1 and Erk2 in combination with DGKz deletion to genetically test the hypothesis that the major effects of DGKz-deficiency in B cells result from elevation of Erk signaling and not other possible signaling effects (Specific Aim 3). Together these three aims will test the hypothesis that DGKz in mature B cells acts primarily via modulating the level of Erk signaling to provide affinity discrimination for B cell activation, expansion, and especially for controlling the numbers of early plasma cells, and in addition will address the biological importance of this regulation.